Tracking systems are used in a wide variety of contexts to provide many different types of information. This information generally regards the location and availability of items being tracked. More specifically, this information includes, but is not limited to, information regarding (1) items received for purchase order reconciliation and accounts payable release purposes; (2) movement of items, storage of items or simply finding items within a facility for inventory control purposes; (3) monitoring the processing of items to ensure that each item has gone through the proper stages of processing; and (4) shipping of items to ensure order correctness and to trigger billing. This kind of information is required in warehouses, distribution centers, manufacturing facilities, service depots, postal sorting facilities, airports, retail and wholesale stores, as well as in any facility or network of facilities, such as a trucking system, which must accurately handle some volume of items.
For example, in a warehouse it is important to keep track of what items are in the warehouse, as well as where items are located within the warehouse. A tracking system in a warehouse should preferably provide information regarding where particular items are stored in the warehouse and how many are stored in the warehouse, as well as information regarding when items are received into the warehouse and when they leave the warehouse.
Different systems have been devised for keeping track of items. This information may, for example, be recorded manually. Alternatively, each item being tracked may be marked with a bar code. In either case, the data must be entered into a central database in order to be useful in tracking items. Bar code scanning automates the entry of data into the central database. Whether a bar code or manual system is used, warehouse personnel must have a clear line of sight to the label of each item in order to either enter the item information manually or to scan the item information into the system using a bar code, and each warehouse worker must also read the labels one at a time. In many cases, packaging of items in boxes, crates, bags or other containers may make this operation very operator-intensive and inaccurate.
Radio frequency identification systems provide a number of advantages over other identification systems, such as manual or bar code systems. In radio frequency identifications systems, information for each item is automatically gathered—warehouse personnel are not required to enter this information. In addition, reading distances can be longer, the tags can be hidden for security reasons, multiple tags can be read simultaneously, and, in the case of read/write tags, information can be stored on the tags, such as the purchase order number or the destination of the item. The foregoing are just some of the advantages of RFID tags.
A typical radio frequency identification system consists of transponders or tags, and an interrogator or reader (or multiple interrogators). The tag may be a single integrated circuit chip bonded to a flat, printed antenna, or could be a complex circuit including a battery and sensors for sensing temperature, position, orientation or other characteristic. RFID tags may be attached to items in many different ways, including being bolted to the item or simply glued to the inside of existing packaging or labeling. RFID tags may be encoded with user-defined data at the time of use, pre-coded with a numbering system at the time of tag manufacturing, or a combination of both approaches may be used.
For a number of reasons, it is preferable to use tags that do not require batteries—such tags are commonly referred to as passive tags and typically receive whatever power they require from an external power source. Passive tags are typically less expensive, require less maintenance, and have extended operating environmental ranges.
In the case of a passive tag, the interrogator will first activate the tag by generating an electromagnetic field of a given frequency. Such an electromagnetic field can be generated, for example, by supplying an alternating electrical current at a given frequency to a coil for low frequency near field systems—commonly called inductively coupled systems—or to an RF antenna for far field higher frequency systems.
The tag includes an antenna—which could be dipole for far field systems or a coil for inductive systems—tuned to the frequency of the electromagnetic field generated by the interrogator. The electrical current thus generated is used to power the tag. Data is generally sent to the tag by modulating the interrogator-generated electromagnetic field, which is commonly called the exciter or illuminating field. The tag can send data back to the interrogator either by transmitting with its own transmitter at a separate frequency from the illuminating field using the antenna, or by modulating the illuminating field by changing the loading of the tag's antenna in what is commonly called a back scatter system. Then, either the new electromagnetic field caused by the tag, or the disturbances in the interrogator's illuminating field caused by the tag's back scatter system, is detected by the interrogator. The data from the tag is then decoded, enabling the tag and the item to which the tag is attached to be identified, and, where the tag is a read/write tag, enabling new or incremental data to be written to the tag.
In a typical RFID tagging application, items to be tracked are tagged and gates with interrogators are installed at various key points in the facility that are significant in the tracking of item. For example, in warehouses, interrogator gates are typically installed at shipping docks in order to read the tags of items before they are loaded onto trucks. This system, however, can lead to many problems.
First, many gates or interrogators must be installed for this system. A typical distribution centre type installation might have more than 30 loading docks, each requiring a separate gate with interrogator. A typical manufacturing or sorting plant type installation might have a huge number of internal checkpoints requiring gates with interrogators in order to track the items and the processing that is applied to each item. Interrogators are very expensive. Thus, an application that requires a lot of interrogators is very expensive.
Second, interrogator gates restrict the freedom of movement of forklifts and people, as well as reducing the floor space available to receive items. This is particularly a problem as gates are likely to be concentrated around the dock doors where space is at a premium—areas adjacent to the dock doors are frequently used as staging areas for temporary storage of items that are being moved onto trucks. The gate spacing and dimensions required to read the tags at the distance, speed, orientation and quantity that items are expected to go through the gate may also greatly restrict movements of the items. This tends to slow down operation and reduce read accuracy, and also makes it likely that the gates and the interrogators will sustain damage.
Typically, a large number of items are loaded on or off a truck or are moved through a facility at a time. For example, it is not uncommon to have 500 tagged items in a bag or box, or on a pallet, being passed through an interrogation gate at any one time. In an anti-collision RFID system, both the interrogator and the tags are specially designed to enable the interrogator to read multiple tags concurrently. In non-collision RFID systems, by way of contrast, only one tag can be in a field at a time in order to ensure a good read. This requirement of anti-collision RFID systems, that the interrogator and tags be designed to enable multiple tags passing through the gate to be read at the same time, slows down the read rate of the interrogators, greatly reduces the accuracy in reading tags, reduces the distances at which the tags can be read, restricts the selection of tag types and frequency, and, in general, raises the question of whether all the tags have been read.
In addition, there may be items stored immediately beside the gate through which a forklift carrying many tagged items is passing, and another interrogator gate may be as little as four feet away at the next loading dock. The very large number of tagged items simultaneously passing through the interrogator gate, the close proximity of other tags that are not passing through the interrogator gate, and the close proximity of another interrogator gate that generates its own electromagnetic field to activate tags passing therethrough, may interfere with an accurate read. Specifically, each interrogator gate is likely to read tags that it should not read—tags that are not passing through the interrogator gate—and to not read tags that it should read—tags that are passing through the interrogator gate.
The way in which items are packed in a container—whether a box, crate, bag or pallet—helps to determine both the orientation of the tag, and the proximity of each tag to other tags. The orientation of a tag relative to the interrogator's antenna is important as a tag whose antenna's polarization is at right angles to that of the interrogator's antenna will be invisible to the interrogator regardless of the distance separating the interrogator from the tag. The proximity of tags to each other is important as two tags that are in close proximity to each other and whose antennae are aligned will steal power from each other, thereby greatly reducing the read range of the two tags as compared to a single tag.
The composition or physical nature of the item being tagged may also significantly reduce the read range and accuracy of the RFID tagging system. An item with a high moisture or metal content can absorb or mask interrogator fields at certain frequencies such that tagged items in the centre of the container, which are surrounded by other items having a high moisture or metal content, cannot be read at all regardless of how slowly they are passed through a gate, or how distant the interrogator is. This may require interrogator fields to operate at frequencies that are inappropriate to the particular application. For example, high frequency interrogator fields are suitable for far field usage (long range applications). However, a bag of water will completely absorb frequencies high enough for far field usage, thereby necessitating the use of inductively coupled tags in situations where such a system is unsuitable for the read range and data speeds required. A person walking past the antennae of either the tags or the interrogator at the time of the read will have the same effect.
The proximity of the interrogator gates to one another, or to radio systems such as RF LAN systems, will also reduce the range and accuracy of the system, and may create confusion between the RF LAN system and the interrogator system. Even a simple, unintentional reading of a tag by a nearby interrogator gate, through which the item is not passing, can destroy the integrity of the data. As a result, tag interrogation systems cannot, in most situations, guarantee a read of all tags at all times. If the accuracy of the interrogation cannot be guaranteed with a high degree of certainty, then the system cannot be used without manual override or intervention. In other words, if it cannot be; determined with confidence that every tag has been read, then manual intervention will be required on every read.
In addition to the foregoing problems, there is an additional problem that many sites within a warehouse are not suitable for installing interrogators, and the mounting problems can be almost as varied as the number of interrogators required, greatly adding to the cost of an already expensive gate or interrogator.
Accordingly, a radio frequency identification system that reduces the number of interrogators required, does not require the interrogators to be located in areas where space is at a premium, such as the docking area, and can better distinguish items that are being moved from items that are not being moved, is desirable. Indeed, a system that can read multiple tags with a high degree of accuracy is required to make an RFID system viable for a wide range of applications.